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Immune system

The body's natural defence mechanism against infectious diseases.


A bacterium, virus, or other microorganism, capable of causing disease.


A list of often difficult or specialised words with their definitions.

What are pathogens?

Microorganisms that cause diseases are called pathogens. They are specialised to infect body tissues where they reproduce and cause damage that gives rise to the symptoms of the infection. The body fights back by mobilising its immune system to fight off the infection. Modern medicine has also developed many successful treatments to assist the body's natural defences.


Types of microorganisms

There are four main types of microorganisms:

In this resource you can find out general information about these microorganisms and you can discover examples of diseases that they cause in the Diseases resource.

Microorganisms in a water sample from a stream

Microorganisms in a water sample from a stream
Courtesy: CDC/ Janice Haney Carr

How to use this site

There are a number of interactive features in this e-source:

  • A glossary of terms: any word with a glossary entry is highlighted like this.
  • Quick questions: at the end of a page/section, there is a set of quick questions to test your understanding of the scientific ideas.
  • Animations: most of the animations can be expanded to full screen size, ideal for showing on an interactive whiteboard. The animations will play all the way through or can be viewed one section at a time.
  • Downloads: Teachers can download individual diagrams, animations and other content from the Download Library area of the website. Terms and Conditions apply.